Microdroplets is a novel means of improving tissue compatibility and providing sustained release of a water insoluble drug by dissolving it into microspheres, (O,1u diameter) of an oil phase and encapsulating them with a layer of phospholipid. A rat model will be used to test the microdroplet technology-based sustained delivery of a narcotic antagonist, naltrexone. Naltrexone is the drug of choice for treatment of heroin addiction because it is essentially a pure narcotic antagonist and devoid of analgesic effects. A series of microdroplet formations containing different concentrations of naltrexone will be prepared in a least three tissue-compatible and FDA approved synthetic oils. The drug release kinetics of the naltrexone-containing microdroplets will be tested in a in vitro model as well as by subcutaneous and intramuscular injections into rats. A formulation which provides acceptable (zero order) release kinetics over a period of 3-4 weeks will be identified for further development in a Phase II SBIR for its potential indication for the treatment of heroin addicts.